The present invention is directed to a method for generating a distortion-free, frequency-modulated optical signal and to apparatus for the implementation of such a method.
In optical heterodyne reception, an optical reference signal having a defined reference frequency in the form of unmodulated light of a local oscillator laser is added in the receiver to a modulated, optical signal generated by an optical transmission laser in the form of modulated light of this transmission laser. Compared to direct reception, higher sensitivity and higher selectivity are simultaneously obtained for the selection of a plurality of offered transmission channels in heterodyne reception.
Binary frequency shift keying (FSK) has proven to be an especially promising modulation method, since the semiconductor lasers employed as signal transmitters can be frequency-modulated in a simple manner by varying the current. Ideally, the achieved frequency shift should be independent of the modulation frequency. This, however, is not normally the case. In particular, sequences that are composed of many identical signals cannot be transmitted distortion-free. Despite a high reception power, the bit error rate frequently cannot be decreased below the value 10.sup.-5 (bit error rate floor).
Known prior art approaches to alleviate this situation are:
Manchester coding (see R. Noe et al., Electronics Letters 25 (1989) 12, pages 757-758) that, due to the doubling of the modulation rate, is not suitable for high bit rates, the same applying to sub-carrier methods;
Alternate mark inversion or AMI coding (see R. Noe et al., Journal of Optical Communications 10 (1989) 3, pages 82-84) that yields lower sensitivity in comparison to normal frequency shift keying;
Bipolar coding (see R. S. Vodhanel, B. Enning, Electronics Letters 24 (1988) 3, pages 163-165), to which the same reason applies as to the AMI coding; and
Quantized feedback from the discriminator to the baseband signal (see B. Enning, R. S. Vodhanel, Electronics Letters 24 (1988) 7, pages 397-399). Given this quantized feedback, characteristic frequency values of the frequency shift keying must not lie in the maximums of an output signal of a frequency discriminator but must lie therebetween so that they can compensate modulation distortions. However, it is known that a sub-optimum, reduced receiver sensitivity results when the characteristic frequency values do not lie in the maximums but therebetween.